Neonatal Hypotonia

Two types of muscle tone can be assessed clinically: postural and phasic. Postural (antigravity) tone is a sustained, low-intensity muscle contraction in response to gravity. It is mediated by both gamma and alpha motor neuron systems in the spinal cord, and it is assessed clinically by passive manipulation of the limbs. Phasic tone is a brief contraction in response to a high-intensity stretch. It is mediated by the alpha motor neuron system only, and is examined clinically by eliciting the muscle stretch reflexes. Hypotonia is defined as reduction in postural tone, with or without a change in phasic tone. When postural tone is depressed, the trunk and limbs cannot overcome gravity, and the child appears hypotonic or floppy.

PHYSICAL EXAMINATION AND ASSESSMENT OF A HYPOTONIC CHILD

After a careful general physical examination, the neurologic assessment needs to include an evaluation of primary neonatal reflexes, a sensory examination, and, most importantly, a motor examination. Muscle tone is assessed by passive manipulation of the infant’s limbs.

Muscle tone can be evaluated further by performing the traction response, vertical suspension, and horizontal suspension maneuvers. A floppy infant exhibits “head lag,” “slips through” the examiner’s hands on vertical suspension, and “drapes over” the examiner’s hand on horizontal suspension.

DIFFERENTIAL ANATOMIC DIAGNOSIS

Neonatal hypotonia may be the manifestation of pathology involving the central nervous system (CNS), the peripheral nervous system (i.e., lower motor unit), or both. In infants with cerebral or central hypotonia (nearly two thirds of these cases), the perinatal or prenatal history may suggest a CNS insult. There may also be associated global (rather than an isolated gross-motor) developmental delay, occasionally seizures, microcephaly, dysmorphic features, and/or malformation of the brain and/or other organs. Central hypotonia may be associated with brisk and/or persistent primitive reflexes and normal-brisk muscle stretch reflexes. The degree of weakness noted in these infants is usually less than the degree of hypotonia (“nonparalytic” hypotonia).

In lower motor unit hypotonia or peripheral hypotonia, developmental delay is primarily gross-motor and is associated with absent or depressed muscle stretch reflexes and/or muscle atrophy and fasciculations of the tongue. In general, antigravity limb movements are decreased and cannot be elicited via postural reflexes. In these infants, the degree of weakness is proportional or in excess of the degree of hypotonia (“paralytic” hypotonia). Trauma to the high cervical cord due to traction in breech or cervical presentation may also initially manifest itself as flaccid paralysis, which may be asymmetric, and initially muscle stretch reflexes are absent; later on, however, upper motor neuron signs develop.

Because muscle tone is also determined by the viscoelastic properties of muscle and joints, connective tissue disorders, such as Marfan and Ehlers-Danlos syndromes, osteogenesis imperfecta, and also benign ligamental laxity, can present with hypotonia. In addition, a combined cerebral and lower motor unit hypotonia occurs in infants and older children as a presenting manifestation of congenital myotonic dystrophy, some congenital muscular dystrophies, peroxisomal disorders, mitochondrial encephalomyopathies, neuroaxonal dystrophy, leukodystrophies (e.g., globoid cell leukodystrophy), familial dysautonomia, and asphyxia secondary to motor unit disease. Further, hypotonia without significant weakness may be a feature of systemic diseases, such as sepsis, congenital heart disease, hypothyroidism, rickets, renal tubular acidosis, and others.

Neuromuscular diseases in infancy manifest primarily with hypotonia and weakness; however, infants with severe hypotonia but only marginal weakness usually do not have a disorder of the lower motor unit (anterior horn cell, peripheral and cranial nerves, neuromuscular junction, and muscle). These infants may have genetic conditions, metabolic disturbances, or as discussed above, systemic disorders (e.g., congenital heart disease, renal failure, etc.).

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